Do you know who said this? Probably many of you do or you figured it out from the picture below. It was Joseph Smith. I have noticed that some people have used this part of the quote to rationalize the worldy view of selfishness and doing what makes you happy, even at the expense of others or of keeping commandments and covenants. Heavenly Father wants me to be happy and I am not happy unless I have my cigarette. I'm not paying tithing because I'm saving for a vacation. I am not happy spending 3 hours in church every week. Wearing beautiful clothes makes me feel good about myself and makes me happy, even if they are immodest.
Heavenly Father does want us happy, but they forget about the rest of the quote.
“Happiness is the object and design of our existence; and will be the end thereof, if we pursue the path that leads to it; and this path is virtue, uprightness, faithfulness, holiness, and keeping all the commandments of God”
Joseph Smith, Jr., Teachings of the Prophet Joseph Smith, sel. Joseph Fielding Smith , 255–56
How does he teach us that we can feel that true happiness? By following the guidelines he gives us to be able to feel the Holy Ghost while we are here, be able to use the atonement of Christ, and to be worthy to return with him. If we seek happiness any other way, it may feel good for a while, but it does not lead to true, lasting happiness. There will be a day when we will be able to see everything in its true light.
Our Heavenly Father wants us to feel real joy, not just in the next life, but now. Real joy can only come from doing what we know He wants us to do and from repenting often of our missteps. We must pursue the only path that leads to that happiness.